The specific absorption rate (SAR) is the most commonly used quantitative index for quantifying the influence on a human body of EM waves radiated by a mobile communication device presently, and is expressed by the following formula:
  SAR  =            σ      ρ        ⁢                                    E          i                            2      
In above formula, σ represents a tissue conductivity (S/m), E represents an electric field strength root mean square value (V/m), and ρ represents a tissue density. It is evident from the formula that the SAR value is positively correlated to the incident electric field strength. When an antenna of the mobile communication device gets very close to the human body, the EM waves radiated by the antenna will make the SAR value get larger, and even exceed the regulation. Therefore, many research institutes adopt various methods to reduce the SAR value at present, so as to reduce the influence on the human body of the EM waves.
There are many methods for reducing the SAR value. Some method is to directly change the structure of the antenna to make the SAR value lower than the regulation. For example, in U.S. Pat. No. 6,958,737 B1, a loop antenna is used to reduce the SAR value, but it may need a large space for this kind of loop antenna.
Some methods are to add an additional element to reduce the SAR value. For example, in U.S. Pat. No. 6,798,168 B2, a copper strip is added to a mobile phone cell to reduce the SAR value; in U.S. Pat. No. 7,672,698 B2, an additional circuit (filter) is added to reduce the SAR value; in U.S. Pat. No. 6,559,803 B2, a dielectric sleeve is added to reduce the SAR value. However, due to the additional elements, although the effect of reducing the SAR value is achieved, the overall performance of the original antenna may usually deteriorate.
Moreover, some methods are to add a barrier between the human body and the antenna to reduce the SAR value. For example, a ferromagnetic material is used (e.g. J. Wang, O. Fujiwara and T. Takagi, “Effects of ferrite patch-shaped attachment to portable telephone in reducing electromagnetic absorption in human head”, IEEE Int. Symp. on Electromagnetic Compatibility, vol. 2, pp. 822-825, 1999), or an electromagnetic band gap (EBG) structure is used (e.g. S. I. Kwak, D. U. Sim, J. H. Kwon and H. D. Choi, “SAR reduction on a mobile phone antenna using the EBG structures”, 38th European Microw. Conf., pp. 1308-1311, October 2008), and a specific split-ring resonator (SRR) structure is used (e.g. J. N. Hwang, and F. C. Chen, “Reduction of peak SAR in the human head with metamaterial”, IEEE Trans. Antennas Propag., vol. 54, no. 12, pp. 3763-3770, December 2006). Although the SAR value could be reduced by above three methods, the performance of the antenna is deteriorated oppositely.
Furthermore, in U.S. Pat. No. 6,421,016 B1, a method is presented for detecting whether a human body gets close in combination with a sensor and switching a current path with a switch to reduce the SAR value, but it is complicated and needs a large space.
In US Patent Publication No. US 2010/0113111 A1, the radiation energy is dispersed and gets away from the human head through guiding, but this technique does not give an overall design for the proximity effect when getting close to the human body, and thus the effect of reducing the SAR value cannot be obtained in the practical use close to the human body. In addition, after the device is installed, the radiation pattern of the antenna is influenced to have a strong directivity, which will impact the signal receiving effect of a handheld communication device.